1. Field of the Invention
The present invention relates to an organic transistor, particularly to a vertical organic transistor structure and a method for fabricating the same.
2. Description of the Related Art
The organic field effect transistor is an indispensable organic electronic element. The structure of the general organic field effect transistor is similar to that of the conventional field effect transistor made of an inorganic semiconductor material, wherein a gate is formed on a substrate; an insulating layer and an organic semiconductor layer are deposited above the gate; then a source and a drain are formed on an identical plane above the organic semiconductor layer. The operating mode of an organic field effect transistor is that an external voltage is applied to the gate to induce a current channel inside the organic semiconductor, and current can thus flow from the source to the drain via the current channel.
As an organic semiconductor generally has a lower carrier mobility than an inorganic semiconductor, it is characterized in low response speed and small current modulation range. To solve the problem, many researches proposed higher carrier mobility organic semiconductors. For example, someone reduced the distance between the source and the drain to increase the drain current. A photolithographic technology is needed to reduce the distance between the source and the drain to a sub-micron dimension. However, the process and solution used in the photolithographic technology are likely to deteriorate organic semiconductors.
Some prior arts proposed schemes to solve the difficulty in obtaining a short channel length between the source and the drain, such as “Self-Aligned, Vertical-Channel, Polymer Field-Effect Transistors” by Stutzmann et al., and “Vertical Channel All-Organic Thin-Film Transistor” by Becker et al., wherein the source and the drain are arranged in vertical direction and separated by an insulating layer; then, a trench is fabricated chemically or mechanically to contain an organic semiconductor layer, an insulating layer and the gate. In such an element, the distance between the source and the drain is determined by the thickness of the insulating layer, and current flows vertically. However, such an element is hard to fabricate because the fabrication of the trench has to use a complex chemical or mechanical method.
Meruvia et al. disclosed a method to change the structure of elements. Similar to a metal base transistor, the element comprises an emitter, a base and a collector, wherein the emitter is made of an organic semiconductor; the base is made of gold; and the collector is made of silicon. This method utilizes the energy gap between the emitter and the base to generate hot electrons and controls the base current to modulate the collector current. However, the collector is made of silicon in this method. Therefore, this method cannot meet the requirement of soft electronics.
A paper “Transistor Electronics: Imperfections, Unipolar and Analog Transistors” by Shockley proposed a scheme: the cathode of a vacuum tube is simulated via forming a heavily-doped cylindrical region in the central portion of a solid semiconductor cylinder; the grid electrode of the vacuum tube is simulated via forming a plurality of heavily-doped cylindrical regions around the cathode; the carrier-receiving plate electrode of the vacuum tube is simulated via forming a heavily-doped cylindrical shell along the perimeter of the solid semiconductor cylinder. As this scheme adopts silicon or germanium as material, it is hard to meet the requirement of soft electronics. Besides, this scheme is also hard to realize with the semiconductor technology.
Yang et al., McElvain et al., and a U.S. Pat. No. 5,563,424 disclosed “Polymer Grid Triodes”, wherein three electrodes are separated by two polymer layers. The operating mode thereof is similar to a vacuum tube: a first electrode injects electrons, and the voltage of a second electrode is used to control the current reaching a third electrode, wherein the current flows in the direction vertical to the substrate. In this design, the second electrode needs treating by a chemical method. Thus, the selectivity of the polymer layer is limited. Besides, the fabrication process thereof is pretty complicated.
Kudo et al. and a U.S. Pat. No. 7,002,176 disclosed a “Vertical Organic Transistor”, wherein three electrodes are separated by two organic layer, and current flows in the direction vertical to the substrate. The second electrode is fabricated with a vapor deposition mask and a dual-source vapor deposition method, wherein the grid electrode geometry formed on a sample is controlled via adjusting the distance between the mask and the sample, the distances between the mask and the vapor deposition sources, the distance between two vapor deposition sources and the distance between the patterns of the mask. The abovementioned method is very complicated and hard to define a very fine boundary of the grid electrode.
A U.S. Pat. No. 6,774,052 disclosed a “Method of Making Nanotube Permeable Base Transistor”, wherein carbon nanotubes are grown on a semiconductor to function as a gate, and a semiconductor material is vapor-deposited on the carbon nanotubes to function as a collector. As this method adopts inflexible materials, it cannot meet the requirement of soft electronics.
A U.S. Pat. No. 6,884,093 disclosed “Organic Triodes with Novel Grid Structures and Method of Production”. As this method needs an expensive semiconductor etching apparatus, the fabrication cost thereof is hard to reduce.
Therefore, the field concerned is desirous to develop an organic field effect transistor, which can be fabricated with a simple process, to implement the next generation soft electronics and display technology.